This invention relates to air depolarized electrochemical cells. This invention is related specifically to metal-air, air depolarized electrochemical cells, especially elongate cylindrical cells. Elongate cells are described herein with respect to cells having the size generally known as xe2x80x9cAA.xe2x80x9d
Button cells, also illustrated herein, are commercially produced in smaller sizes having lesser height-to-diameter ratios, and are generally directed toward use in hearing aids, and computer applications. Such button cells generally feature overall contained cell volume of less than 2 cm3, and for the hearing aid cells less than 1 cm3.
The advantages of air depolarized cells have been known as far back as the 19th century. Generally, an air depolarized cell draws oxygen from air of the ambient environment, for use as the cathode active material. Because the cathode active material need not be carried in the cell, the space in the cell that would have otherwise been required for carrying cathode active material can, in general, be utilized for containing anode active material.
Accordingly, the amount of anode active material which can be contained in an air depolarized cell is generally significantly greater than the amount of anode active material which can be contained in a 2-electrode cell of the same overall size. By xe2x80x9c2-electrodexe2x80x9d cell, we mean an electrochemical cell wherein the entire charge of both anode active material and cathode active material are contained inside the cell structure when the cell is received by the consumer.
Generally, for a given cell size, and similar mass, an air depolarized cell can provide a significantly greater number of watt-hours of electromotive force than can a similarly sized, and similar mass, 2-electrode cell using the same, or a similar, material as the anode electroactive material.
Several attempts have been made to develop and market commercial applications of metal-air cells. However, until about the 1970""s, such cells were prone to leakage, and other types of failure.
In the 1970""s, metal-air button cells were successfully introduced for use in hearing aids, as replacement for 2-electrode cells. The cells so introduced were generally reliable, and the incidence of leakage had generally been controlled to an extent sufficient to make such cells commercially acceptable.
By the mid 1980""s, zinc-air cells became the standard for hearing aid use. Since that time, significant effort has been made toward improving metal-air hearing aid cells. Such effort has been directed toward a number of issues. For example, efforts have been directed toward increasing electrochemical capacity of the cell, toward consistency of performance from cell to cell, toward control of electrolyte leakage, toward providing higher voltages desired for newer hearing aid appliance technology, toward higher limiting current, and toward controlling movement of moisture into and out of the cell, and the like.
An important factor in button cell performance is the ability to consistently control movement of the central portion of the cathode assembly away from the bottom wall of the cathode can during final cell assembly. Such movement of the central portion of the cathode assembly is commonly known as xe2x80x9cdoming.xe2x80x9d
Another important factor in button cell performance is the electrical contact between the cathode current collector and the cathode can or other cathode terminal. Conventional cathode current collectors comprise woven wire screen structure wherein ends of such wires provides the electrical contact between the cathode current collector and the inner surface of the cathode can.
While metal-air button cells have found wide-spread use in hearing appliances, and some use as back-up batteries in computers, air depolarized cells have, historically, not had wide-spread commercial application for other end uses, or in other than small button cell sizes.
The air depolarized button cells readily available as items of commerce for use in hearing aid appliances are generally limited to sizes of no more than 0.6 cm3 overall volume. In view of the superior ratio of xe2x80x9cwatt-hour capacity/massxe2x80x9d of air depolarized cells, it would be desirable to provide air depolarized electrochemical cells in additional sizes and configurations, and for other applications. It would especially be desirable to provide air depolarized electrochemical cells which are relatively much larger than button cells. For example, it would be desirable to provide such cells in xe2x80x9cAAxe2x80x9d size as well as in the standard button cell sizes.
It is an object of the invention to provide an elongate an depolarized electrochemical cell having a cathode extending along the length of the cell, and having a bottom electrically insulating seal member extending across the bottom of the anode cavity between the anode cavity and the bottom wall of the cell.
It is another object of the invention to provide an air depolarized cell wherein the bottom seal member comprises a melt plug.
It is a further object to provide such a cell having such a melt plug by melting one or more solid particles of electrically insulating thermoplastic seal material inside the anode cavity.
It is a yet further object to provide a cell having such a melt plug by melting such solid particles by driving heat through the bottom wall of the cell.
Still another object is to provide a cell having such a melt plug by melting such solid particles by applying heat to the solid particles by applying heat through the top of the cell.
Another object is to provide a cell having such a melt plug by spray-applying a melted such thermoplastic electrically-insulating material into the bottom of the otherwise-empty anode cavity.
Another object is to provide a cell wherein the bottom seal member comprises an isolation cup extending generally across the bottom of the anode cavity and separating the anode material from the bottom wall.
A yet further object is to provide a cell wherein the bottom seal member comprises a combination of an isolation cup between the bottom wall and a melt plug.
A still further object is for the separator to extend below the top of the bottom seal member at the joint between the bottom seal member and the separator, and to terminate above the bottom of the cathode current collector.
It is yet another object to provide a method of fabricating an elongate air depolarized cell, including melting one or more particles of an electrically insulating thermoplastic seal material in situ in the bottom of the anode cavity, and subsequently to solidify the melt plug material, thereby to provide an insulating melt plug between the positively-charged bottom wall and the negatively-charged anode material.
In a first family of embodiments, the invention comprehends an elongate air depolarized electrochemical cell. The cell has a top and a bottom, and a transverse cross-section disposed along a length of the cell. The cell comprises a cathode, including an air cathode assembly extending along the length of the cell; an anode, including an anode cavity and electroactive anode material in the anode cavity, the anode cavity having a top and a bottom; and a separator, having an inner side wall thereof defining a side wall of the anode cavity between the anode material and the cathode assembly. Electrolyte is dispersed in the anode, the cathode, and the separator. A bottom closure member is electrically connected to the cathode assembly and forms a bottom wall of the air depolarized electrochemical cell. An electrically insulating bottom seal member extends generally across that portion of the transverse cross-section which spans the anode cavity at the bottom of the anode cavity. The bottom seal member separates the electroactive anode material from the bottom wall, and provides a seal about the side wall of the anode cavity at the separator.
In preferred embodiments, the bottom seal member comprises a melt plug extending generally across the bottom of the anode cavity, electrically separating the electroactive anode material from the bottom wall. The melt plug preferably results from placing one or more particles of thermoplastic material at the bottom of the anode cavity, and subsequently melting the one or more particles in situ and thereby activating melt flow of the thermoplastic material thus to cove that portion of the transverse cross-section which spans the bottom of the anode cavity. The melt plug preferably comprises an upwardly-inclined meniscus at the separator.
In preferred embodiments the melt plug results from driving heat through the bottom wall of the bottom closure member and thereby melting one or more particles of thermoplastic seal material in the bottom of the anode cavity.
In other embodiments, the melt plug results from application of an already melted thermoplastic material in melt form in the bottom of the anode cavity such that the melted material reaches at least as high as an equal quantity of such material would reach by surface tension in a melted state.
Whatever the form of the bottom seal member, and however the bottom seal member is generated and/or applied in the bottom of the anode cavity, in preferred embodiments, the cell includes a downwardly extending slot adjacent an outer side wall of the bottom closure member, the cathode assembly extending downwardly into the slot and making electrical connection with the bottom closure member in the slot. Preferably, the separator extends downwardly below the top of the melt plug, as established adjacent the separator, and terminates above the bottom edge of the cathode current collector.
Either alone or in combination with a melt plug, the bottom seal member can comprise an electrically insulating isolation cup extending across the cross-section of the cell at the bottom of the anode cavity, between the anode material and the bottom wall, electrically separating the electroactive anode material from the bottom wall, and providing a seal about the side wall of the anode cavity at the separator. Where an isolation cup is used in combination with a melt plug, the isolation cup is generally between the melt plug and the bottom wall of the cell.
The invention further comprehends a method of fabricating an elongate air depolarized electrochemical cell having a top, a bottom, and a transverse cross-section disposed along a length of the cell. The cell comprises a cathode including a cathode assembly, an anode including an anode cavity containing electroactive anode material, a separator having an inner side wall thereof defining a side wall of the anode cavity between the electroactive anode material and the cathode assembly, and electrolyte dispersed in the anode material, the cathode assembly, and the separator. A bottom closure member is electrically connected to the cathode assembly and forms a bottom wall of the cell. The method comprises assembling a cathode assembly to a bottom closure member and thereby defining the anode cavity inwardly of the separator; and before loading electroactive anode material into the anode cavity, placing one or more particles of thermoplastic seal material into the anode cavity and subsequently melting the one or more particles in situ and thereby activating melt flow of the thermoplastic seal material outwardly to the separator such that the melted thermoplastic seal material extends generally across that portion of the transverse cross-section which spans the anode cavity and develops a melt plug separating the anode cavity from the bottom wall of the bottom closure member. The method further comprises solidifying the melted thermoplastic seal material to form a solid electrically insulating melt plug separating the anode cavity from the bottom wall, subsequently loading electroactive anode material into the anode cavity, and after loading the electroactive anode material into the anode cavity, employing top closure structure at the top of the cell to thereby close the cell.
Typically, the melting of the one or more particles includes developing an upwardly-inclined meniscus in the melted thermoplastic material adjacent the separator.
In some embodiments, the melting of the one or more particles comprises driving heat through the bottom wall of the bottom closure member to the one or more particles of seal material. In other embodiments, the melting of the particles comprises applying heat to the one or more particles through an open top of the anode cavity.